Gas turbine engines have a turbine where hot gas is expanded to gather mechanical work. Typically the turbine has a plurality of stages, each comprising vanes (which do not rotate) and blades (which rotate).
The blades have to withstand very severe conditions, due for example to the high centrifugal forces and the high temperature of the gas they are immersed in. The conditions are particularly severe for long blades, such as the blades of the last stages (e.g. third, fourth or subsequent stages) of the turbine, because of the particularly high centrifugal forces.
In order to provide blades able to withstand severe conditions, blades made of ceramic matrix composite material (CMC) have been proposed. CMC is a composite material having carbon or ceramic fibers and a ceramic matrix. US 2012/0 195 766 A1 discloses a blade of this kind.
In particular, in the following reference is made to blades whose root has a shell structure; a shell structure is to be understood as a hollow structure having walls made of CMC. The airfoil can have a shell structure as well or it can have a solid structure; the airfoil is advantageously made of CMC.
A problem with these kinds of blades is the connection of the blades to the rotor. In fact, due to the high stress during operation, there is the risk that the hollow structure of the root collapses.